This invention relates to a method for continuously producing reinforced laminates.
Resin-impregnated reinforced laminates such as clad or unclad laminates of electrical grade have been produced by impregnating fibrous reinforcing sheets with a resin solution, drying and partially curing the impregnated material to form so-called "prepreg", stacking the prepreg to a desired thickness and heating the stack under pressure. This process is carried out batchwise and requires unduly large amounts of labor and skill. Attempts have been made to effect the foregoing steps on continuous basis. Difficulty is present, however, in continuously carring out the final curing step in which plies of prepregs are heated under pressure for a length of time. Great pressure is required in this step to finish to a product having a uniform thickness and smooth surface characteristics.
U.S. Pat. No. 2,596,162 to Muskat describes a technique for the continuous production of a fiber-reinforced resin laminate wherein a stack of impregnated substrate sheets is sandwiched between a pair of non-porous flexible sheets (such as cellophane) and cured as such without the application of pressure thereto. However, this references does not teach the use of a cladding metal foil, which becomes an integral part of the finished laminate, as a covering sheet for the uncured laminate during the curing process. Furthermore, in the process of the reference, the polymerization of the resin component used for impregnation of the substrate is discontinued or interrupted before the final polymer is obtained, preferably while the polymer is tacky or adhesive. While a metal foil is incidentally mentioned as a suitable cover sheet, the reference specifically teaches that said materials are normally non-adherent to the polymer so that they may be removed readily after the resin impregnated sheets are cured.
As a result of interrupting polymerization when an adhesive state is reached, the semi-cured laminate prepared by this prior art method is then cut into sections which in turn are fully cured while holding them in the desired final shape. Such a process is not suitable for the production of metal clad laminates, wherein processing while the resin is in a semi-solid state must be avoided because the metal foil is not secured in position.
U.S. Pat. No. 3,810,816 to Zachariades shows a technique whereby the metal foil becomes an integral part of the finished laminate. The foil used in this prior art technique serves not only as a cover sheet, but also as a means for applying substantial pressure on the impregnated substrate. In the prior art method, the foil is naturally subjected to bending forces at various points after it has been applied on the impregnated substrate; this is only possible when the finished laminate is sufficiently flexible and thin. When a bending force is applied on a laminate in a direction perpendicular to the plane thereof, the outermost surface is pulled circumferentially, while the inner surface is similarly compressed. The greater of the thickness of the laminate, the greater the displacement of the surface area thereon. Metal clad, rigid laminates are apparently unable to accommodate such repeated bendings.